in the mass movement. The major effect of ice
melt on the mobility of rock and ice avalanches
is documented ( 9 , 10 ), but it appears that the
combination of the specific rock/ice fraction
(~80/20% by volume) and large fall height of
the rock and ice avalanche led to a rare, severe
event during which nearly all of the ice melted.
Soon after the disaster, media reports and
expert opinions started to circulate, postulat-
ing links of the event to climate change. Recent
attribution studies demonstrated that glacier
mass loss on global, regional, and local scales is
to a large extent attributable to anthropogenic
greenhouse gas forcing ( 36 , 37 ). High-mountain
slope failures in rock and ice, however, pose
additional challenges to attribution owing to
multiple factors and processes involved in
such events. Although long-term trends of
increasing slope failure occurrence in some
regions could be attributed to climate change
( 16 , 38 , 39 ), attribution of single events such
as the Chamoli event remains largely elusive.
Nevertheless, certain elements of the Chamoli
event have potential links to climate and
weather, as described below. Furthermore, the
Chamoli event may be seen in the context of
a change in geomorphological sensitivity ( 40 )
and might therefore be seen as a precursor for
an increase in such events as climate warming
proceeds.
The stability of glacierized and perennially
frozen high-mountain slopes is indeed partic-
ularly sensitive to climate change ( 16 ). Our
analysis suggests regional climate and related
cryospheric change could have interacted in a
complex way with the geologic and topographic
setting to produce this massive slope failure. Air
and surface temperatures have been increasing
across the Himalayan region, with greater rates
of warming during the second half of the 20th
century and at higher elevations ( 41 , 42 ). Most
glaciers in the Himalaya are shrinking, andmass loss rates are accelerating across the re-
gion [( 22 ), section 5.4, and ( 43 – 46 )]. Glacier
shrinkage uncovers and destabilizes mountain
flanks and strongly alters the hydrological and
thermal regimes of the underlying rock.
The detachment zone at Ronti Peak is about
1 km higher than the regional lower limit of
permafrost at around 4000 to 4500 m above
sea level, as indicated by rock glaciers in the
region and global permafrost maps ( 47 , 48 ).
Exposed rock on the north face of Ronti Peak
likely contains cold permafrost, with rock
temperatures several degrees below 0°C. In
connection with glaciers, however, ground tem-
peratures can be locally higher. The ice-free
south face of Ronti Peak is certainly substan-
tially warmer, with rock temperatures perhaps
around or above 0°C, causing strong south-to-
north lateral heat fluxes ( 49 ). Permafrost tem-
peratures are increasing worldwide, particularly
in cold permafrost ( 16 , 50 , 51 ), leading to long-
term and deep-seated thermal anomalies and
even permafrost degradation ( 49 ). Increasing
ground temperatures at the failure site of the
Chamoli avalanche could have resulted in re-
duced strength of the frozen rock mass by al-
tering the rock hydrology and the mechanical
properties of discontinuities and the failed
rock mass ( 52 ).
The geology of the failed rocks includes sev-
eral observed or inferred critical attributes
[( 22 ), section 1]: (i) The rocks are cut by mul-
tiple directions of planar weaknesses; the failed
mass detached along four of these. (ii) The rock
mass is close to a major thrust fault, with many
local shear fractures, which—along with other
discontinuities—would have facilitated aqueous
chemical weathering. (iii) The rock types (schist
and gneiss), even when nominally unweathered,
contain abundant soft, platy, oriented, and geo-
mechanically anisotropic minerals (phyllosi-
licates and kyanite especially); weathering will
further weaken these rocks, and they will be
more likely to disintegrate into fine material
upon impact, which would influence the rheol-
ogy and likely enhance the mobility of the
mass flow.
The 7 February failure considerably changed
the stress regime and thermal conditions in
the area of the detachment zone. Only detailed
investigations and monitoring will determine
whether rock or ice adjacent to the failed block
(including a large hanging rock block above
the scarp) were destabilized because of these
changes and present an ongoing hazard. Sim-
ilarly, the impoundment at the Ronti Gad–
Rishiganga River confluence requires careful
monitoring because embedded ice in the dam
deposits may melt with warmer temperatures,
increasing the risk of an outburst flood by re-
ducing lake freeboard of the dam, and/or re-
ducing structural coherence of the dam.
Videos of the event, including the ones
broadcast on social media in real time [( 22 ),304 16 JULY 2021•VOL 373 ISSUE 6552 sciencemag.org SCIENCE
Fig. 4. Flow evolution scenarios and simulation.(A) Schematic of the evolution of the flow from the
source to Tapovan. (B) Maximum flow height simulated with r.avaflow, showing the observed trim lines for
comparison. P0 is the location of the velocity estimate derived from seismic data, and P1 to P4 are locations
of velocity estimates based on videos and satellite images. (C) Along-profile evolution of flow velocity
and fractions of rock/debris, ice, and water simulated with r.avaflow.“Front”refers to the flow front, whereas
“main”refers to the point of maximum flow momentum. (D) Simulated and estimated peak discharges
and travel times at above locations.
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